Method of nitriding



METHOD OF NITRIDING Filed Dec. 22, 1965 lst. Step Preferred Range lst.Step 200'. Step 55 kzmummm ACTIVITY or NITROGEN, PNHJ/PHZJ/Z (P inAtmospheres) 550 560 570 TEMPERATURE, 0.

United States PatentO 3,399,085 METHOD OF NITRIDING Herbert E. Knechtel,Monroeyille Borough, and Harry H. Podgurski, Greensburg, Pa., assignorsto United States Steel Corporation, a corporation of Delaware FiledDec.22, 1965, Ser. No. 515,548 6 Claims. (Cl. 148-16.6)

ABSTRACT OF THE DISCLOSURE Method of nitriding steel surfaces inwhichsurface is treated with a binary mixture of ammonia 'and hydrogenat an elevated temperature and atmospheric pressure. The ammoniadecomposes and its nitrogen combines with alloying elements in the steel,to form nitrides. Conditions are controlled to prevent nucleation ofiron nitride, thus avoiding formation of objectionable White layer,which otherwise must be machined off the nitrided surface.

This invention relates to an improved method of nitriding steel.

It is known that a hardened case can be formed on the surface of certaintypes of steel by 'a nitriding process. Steels suitable for nitridingcontain several alloyed elements, commonly aluminum, chromium andmolybdenum, and sometimes nickel or vanadium. The conventional procedureis to pass cracked ammonia over the surface of a nitriding steel at atemperature of about 520 to 560 C. Some of the ammonia decomposes on thesurface of the steel, and nitrogen thus released diffuses into the solidphase and combines with some of these alloyed elements to form nitrides.After a sufficiently long treatment, the nitrides produce a hardenedcase to a depth of several thousandths of 'an inch. Any molecularnitrogen (N generated during nitriding or present initially in thecracked ammonia atmosphere has such low nitrogen activity (hereinafterdefined) that it can be ignored as a nitriding agent. The presence of Nneed only be considered in calculating the effective nitrogen activityof the nitriding atmosphere insofar as it affects calculations ofpartial pressures of NH and H Previous nitriding processes wereconducted under conditions which produce a brittle outer skin on thecase, known as white layer or damage. In the white layer some ironnitride is formed, along with nitrides of the alloyed elements. Withinthe rest of the case, only the alloyed elements form nitrides. Becausethe white layer tends to spall, it is often removed, as by grinding orchemical treatment, before the nitrided part is placed in service. Formany pumposes a nitrided case is prepared with a hardness of at least1000 DPH and a depth of at least 20 mils. The belief has been that acase of this hardness and depth can be obtained only under conditionswhich form 'a white layer. For a more extended discussion of thisproblem, reference may be made to Floe Patent No. 2,437,249.

An object of our invention is to p rovide an improved method ofnitriding steel surfaces in which we obtain a case of hardness and depthat least as great as in previous methods, but we avoid altogetherformation of a white layer.

A more specific object is to provide an improved nitriding method whichwe may conduct in either one or two stepse and in which we circulateammonia-hydrogen mixtures over the surface of a nitriding steel, butcontrol both temperature and nitrogen activity throughout the process toavoid formation of a white layer, yet obtain a case with desiredcharacteristics.

In the drawing:

FIGURE 1 is a graph which shows the relation be- "ice 2 tween nitrogenactivity and composition of (A) binary gas mixtures of NH and H (B)dissociated NH, at different volume percentages of dissociation, and (C)dissociated NH at different volume percentages of N2+H2; and v v,

FIGURE 2 is a graph which specifies the nitrogen activity we maintain inthe nitriding gas atmosphere for any temperature we use during theoptional second step of our method.

We have observed that the most reliable guide-to the nitriding power of.a gas formed by dissociation of ammonia is its nitrogen activityasdetermined by the formula: Y Nitrogen activity:

Partial pressure of NH in atmospheres (Partial pressure'of H inatmospheres) For example, the nitrogen activity of a binary mixturecomposed of 50% by volume NH and 50% H at one atmosphere pressure is:

A mixture of NH H and N at one atmosphere formed when parts of NH are50% dissociated according to the reaction:

consists of 50 parts by volume NH 75 parts H and 25 parts N parts inall. The partial pressures P are represented by the fractions:

The nitrogen activity of this gas mixture at one atmosphere pressure is:

The nitrogen activity of a gas which consists of 50% by volume NH and50% by volume H +N as dissociation products of cracked ammonia at oneatmosphere absolute pressure is:

In the last example P has the value shown for the reason that Hconstitutes 75% by volume of the product for each mole of NH which iscracked.

In FIGURE 1 curve A shows the nitrogen activity of different binarymixtures of NH and H at one atmosphere pressure. Curve B shows thenitrogen activity for dissociated NH at different percentages ofdissociation, likewise at one atmosphere pressure. Curve C shows thenitrogen activity for different volume percentages of N +H in adissociated NH mixture at one atmosphere pressure. (Volume percent of N+H is commonly but erroneously referred to as the degree of NHdissociation.) To produce a nitrogen activity of any chosen value, weuse as a nitriding gas a binary mixture of NH, and H in a volume ratiowhich may be determined from curve A.

According to our method we nitride 'a steel surface under conditionswhich altogether avoid iron nitride nucleation on the surface of thesteel. We begin our nitriding operation by passing over the steelsurface, at substantially atmospheric pressure and at a selectedtemperature between about 475 and 530 C., a binary mixture of NH, and Hwhich has a nitrogen activity of about 0.5 to 1.8. By reference to curveA of FIGURE 1 it is seen that a binary mixture which consists of about30 to 55 percent by volume NH and the remainder H possesses a nitrogenactivity within this range. For each specific combination of temperatureand nitrogen activity and each. specific steel, we continue thistreatment for a time 'a little shorter than that which leads tonucleation of a white layer. We determine the nucleation timeexperimentally. For example, with one commonly used nitriding steel(Nitralloy 135M), if we operateat the upper limits of temperature (530C.) and nitrogen activity (1.8), we must limit the nitriding time toless than 17 hours to avoid white layer completely. At lowertemperatures or lower nitrogen activities, we can prolong the treatmentproportionally.

Our preferred temperature is about 500 C., and our preferred nitrogenactivity about 0.6 to 1.8 (34 to 55 percent by volume NH in the binarygas mixture). At 500 C. and a nitrogen activity of 1.17, we must ceasenitriding the aforementioned steel within about 40 hours. A 30-hourtreatment under such conditions produced a case which has a surfacehardness of about 1100 DPH and 'a depth of about 16 mils. At 500 C. anda nitrogen activity of 0.6, we find no limiting nitriding time up to 260hours. A 65-hour treatment of the same steel under the last mentionedconditions produced a case which has a surface hardness of about 1100DPH and a depth of about 18 mils. At 260 hours we attained a surfacehardness of about 1100 DPH and a case depth of about 30 mils.

The single nitriding step just described produces a case-hardened steelwhich may be used as a finished product for many applications. If only ashallow case is needed, we can obtain a useful product under any set ofconditions within our broader limits, as long as we stop before wenucleate a white layer. If a deeper case is needed, we can operatewithin our preferred limits and prolong the treatment. Thus oursingle-step treatment can produce a case of any desired depth only atthe expense of a rather long nitriding time. To obtain a case ofcomparable depth in a shorter time, we can of course change theconditions. during this step, starting at a higher nitrogen activity,and lowering it before we nucleate a white layer, but staying within thebroader limits. Preferably we may use a two-step method.

In the optional second step of our method, we further treat thepartially nitrided surface with a binary mixture of NH;; and H at ahigher temperature but at a lower nitrogen activity. In this step wetreat the surface at substantially atmospheric pressure under conditionswhich approach, but lie below, the curve shown in FIG- URE 2. That is,we employ nitrogen activities between about 0.16 and 0.21 (11 to percentby volume NH in the binary mixture) and operate at a selectedtemperature between about 580 and 540 C. as determined from the curve.We have continued this treatment for as long as 440 hours withoutproducing white layer, while obtaining a case depth of up to about 60mils.

As an example to demonstrate how our invention operates, we employed atwo-step method to nitride inch thick sections of a nitriding steelNitralloy 135M) of the following composition:

Iron Remainder In the first step we used a nitrogen activity of 1.17 (46percent NH 54 percent H in the binary mixture) at a temperature of 500C. A 20-hour treatment yielded a case 16 mils in depth and having asurface hardness of 1100 DPH. This steel could be used as a finishedproduct. In the second step we used a nitrogen activity of 0.167 (14percent NH 86 percent H in the binary mixture) at 'a temperature of 570C. A 90-hour treatment increased the case depth to 30 mils. Extendingthe time in the second step to 212 hours, maintaining all otherconditions, increased the case depth to 44 mils, still without whitelayer.

From the foregoing description it is seen that our invention affords asimple effective method of nitriding v and avoiding formation of whitelayer. Since we never form such a layer, we eliminate costly steps ofmachining or chemical treatment to remove it. The surface of the caseremains as bright as it was before nitriding.

While we have shown and described certain preferred embodiments of ourinvention, it is apparent that other modifications may arise. Therefore,we do not wish to be limited to the disclosure set forth but only by thescope of the appended claims.

We claim:

1. A method of nitriding the surface of 'a nitriding steel comprisingpassing over the surface, at substantially atmospheric pressure and at atemperature of about 475 to 530 C., a binary mixture of NH and H havinga nitrogen activity of about 0.5 to 1.8 until a case is formed having ahardness of at least 1000 DPH and a depth of at least 16 mils, butterminating this step before nucleating a white layer.

2. A method as defined in claim 1 in which the binary mixture of gasesconsists of about 30 to 55 percent by volume NH and the remaindersubstantially H 3. A method as defined in claim -1 in which the nitrogenactivity is about 0.6 to 1.8 and the temperature about 500 C.

4. A method as definid in claim 3 in which the binary mixture of gasesconsists of about 34 to 55 percent by volume NH and the remaindersubstantially H 5. A method as defined in claim 1 wherein the stepdefined constitutes the sole nitriding treatment.

6. A method as defined in claim 1 comprising in addition a secondnitriding step which consists in passing over the surface, atsubstantially atmospheric pressure and at a temperature of about 540 to570 C., a binary mixture of NH and H having an upper limit of nitrogenactivity fixed :by the curve shown in FIGURE 2.

References Cited UNITED STATES PATENTS 2,437,249 3/1948 Floe 148166OTHER REFERENCES Metal Progress, December 1946, The Nitriding of Steel,pp. 1212-1220.

Kohaszati Lapok, 1956, pp. 199-206.

CHARLES N. LOVELL, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,399,085 August 27 1968 Herbert E. Knechtel et al.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 1, line 52, "pumposes" should read purposes line 66, "stepse"should read steps Column 2, line 31,

should read P 50/ Signed and sealed this 27th day of January 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

